/*
 * Copyright (c) 2010, 2020 Oracle and/or its affiliates. All rights reserved.
 *
 * This program and the accompanying materials are made available under the
 * terms of the Eclipse Public License v. 2.0, which is available at
 * http://www.eclipse.org/legal/epl-2.0.
 *
 * This Source Code may also be made available under the following Secondary
 * Licenses when the conditions for such availability set forth in the
 * Eclipse Public License v. 2.0 are satisfied: GNU General Public License,
 * version 2 with the GNU Classpath Exception, which is available at
 * https://www.gnu.org/software/classpath/license.html.
 *
 * SPDX-License-Identifier: EPL-2.0 OR GPL-2.0 WITH Classpath-exception-2.0
 */

package org.glassfish.grizzly.utils;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.lang.reflect.Array;
import java.net.URL;
import java.util.ArrayList;
import java.util.Enumeration;
import java.util.Iterator;
import java.util.List;
import java.util.NoSuchElementException;
import java.util.Set;
import java.util.TreeSet;

/**
 * A simple service-provider lookup mechanism. A <i>service</i> is a well-known set of interfaces and (usually abstract)
 * classes. A <i>service provider</i> is a specific implementation of a service. The classes in a provider typically
 * implement the interfaces and subclass the classes defined in the service itself. Service providers may be installed
 * in an implementation of the Java platform in the form of extensions, that is, jar files placed into any of the usual
 * extension directories. Providers may also be made available by adding them to the applet or application class path or
 * by some other platform-specific means.
 * <p/>
 * <p>
 * In this lookup mechanism a service is represented by an interface or an abstract class. (A concrete class may be
 * used, but this is not recommended.) A provider of a given service contains one or more concrete classes that extend
 * this <i>service class</i> with data and code specific to the provider. This <i>provider class</i> will typically not
 * be the entire provider itself but rather a proxy that contains enough information to decide whether the provider is
 * able to satisfy a particular request together with code that can create the actual provider on demand. The details of
 * provider classes tend to be highly service-specific; no single class or interface could possibly unify them, so no
 * such class has been defined. The only requirement enforced here is that provider classes must have a zero-argument
 * constructor so that they may be instantiated during lookup.
 * <p/>
 * <p>
 * A service provider identifies itself by placing a provider-configuration file in the resource directory
 * <tt>META-INF/services</tt>. The file's name should consist of the fully-qualified name of the abstract service class.
 * The file should contain a list of fully-qualified concrete provider-class names, one per line. Space and tab
 * characters surrounding each name, as well as blank lines, are ignored. The comment character is <tt>'#'</tt>
 * (<tt>0x23</tt>); on each line all characters following the first comment character are ignored. The file must be
 * encoded in UTF-8.
 * <p/>
 * <p>
 * If a particular concrete provider class is named in more than one configuration file, or is named in the same
 * configuration file more than once, then the duplicates will be ignored. The configuration file naming a particular
 * provider need not be in the same jar file or other distribution unit as the provider itself. The provider must be
 * accessible from the same class loader that was initially queried to locate the configuration file; note that this is
 * not necessarily the class loader that found the file.
 * <p/>
 * <p>
 * <b>Example:</b> Suppose we have a service class named <tt>java.io.spi.CharCodec</tt>. It has two abstract methods:
 * <p/>
 * 
 * <pre>
 * public abstract CharEncoder getEncoder(String encodingName);
 * 
 * public abstract CharDecoder getDecoder(String encodingName);
 * </pre>
 * <p/>
 * Each method returns an appropriate object or <tt>null</tt> if it cannot translate the given encoding. Typical
 * <tt>CharCodec</tt> providers will support more than one encoding.
 * <p/>
 * <p>
 * If <tt>glassfish.io.StandardCodec</tt> is a provider of the <tt>CharCodec</tt> service then its jar file would
 * contain the file <tt>META-INF/services/java.io.spi.CharCodec</tt>. This file would contain the single line:
 * <p/>
 * 
 * <pre>
 *   glassfish.io.StandardCodec    # Standard codecs for the platform
 * </pre>
 * <p/>
 * To locate an codec for a given encoding name, the internal I/O code would do something like this:
 * <p/>
 * 
 * <pre>
 * CharEncoder getEncoder(String encodingName) {
 *     for (CharCodec cc : ServiceFinder.find(CharCodec.class)) {
 *         CharEncoder ce = cc.getEncoder(encodingName);
 *         if (ce != null)
 *             return ce;
 *     }
 *     return null;
 * }
 * </pre>
 * <p/>
 * The provider-lookup mechanism always executes in the security context of the caller. Trusted system code should
 * typically invoke the methods in this class from within a privileged security context.
 *
 * @author Mark Reinhold
 * @version 1.11, 03/12/19
 * @since 1.3
 */
public final class ServiceFinder<T> implements Iterable<T> {

    private static final String prefix = "META-INF/services/";

    private final Class<T> serviceClass;
    private final ClassLoader classLoader;

    /**
     * Locates and incrementally instantiates the available providers of a given service using the given class loader.
     * <p/>
     * <p>
     * This method transforms the name of the given service class into a provider-configuration filename as described above
     * and then uses the <tt>getResources</tt> method of the given class loader to find all available files with that name.
     * These files are then read and parsed to produce a list of provider-class names. The iterator that is returned uses
     * the given class loader to lookup and then instantiate each element of the list.
     * <p/>
     * <p>
     * Because it is possible for extensions to be installed into a running Java virtual machine, this method may return
     * different results each time it is invoked.
     * <p>
     *
     * @param service The service's abstract service class
     * @param loader The class loader to be used to load provider-configuration files and instantiate provider classes, or
     * <tt>null</tt> if the system class loader (or, failing that the bootstrap class loader) is to be used
     * @throws ServiceConfigurationError If a provider-configuration file violates the specified format or names a provider
     * class that cannot be found and instantiated
     * @see #find(Class)
     */
    public static <T> ServiceFinder<T> find(Class<T> service, ClassLoader loader) {
        return new ServiceFinder<>(service, loader);
    }

    /**
     * Locates and incrementally instantiates the available providers of a given service using the context class loader.
     * This convenience method is equivalent to
     * <p/>
     * 
     * <pre>
     * ClassLoader cl = Thread.currentThread().getContextClassLoader();
     * return Service.providers(service, cl);
     * </pre>
     *
     * @param service The service's abstract service class
     *
     * @throws ServiceConfigurationError If a provider-configuration file violates the specified format or names a provider
     * class that cannot be found and instantiated
     * @see #find(Class, ClassLoader)
     */
    public static <T> ServiceFinder<T> find(Class<T> service) {
        return find(service, Thread.currentThread().getContextClassLoader());
    }

    private ServiceFinder(Class<T> service, ClassLoader loader) {
        this.serviceClass = service;
        this.classLoader = loader;
    }

    /**
     * Returns discovered objects incrementally.
     *
     * @return An <tt>Iterator</tt> that yields provider objects for the given service, in some arbitrary order. The
     * iterator will throw a <tt>ServiceConfigurationError</tt> if a provider-configuration file violates the specified
     * format or if a provider class cannot be found and instantiated.
     */
    @Override
    public Iterator<T> iterator() {
        return new LazyIterator<>(serviceClass, classLoader);
    }

    /**
     * Returns discovered objects all at once.
     *
     * @return can be empty but never null.
     *
     * @throws ServiceConfigurationError
     */
    @SuppressWarnings("unchecked")
    public T[] toArray() {
        List<T> result = new ArrayList<>();
        for (T t : this) {
            result.add(t);
        }
        return result.toArray((T[]) Array.newInstance(serviceClass, result.size()));
    }

    private static void fail(Class service, String msg, Throwable cause) throws ServiceConfigurationError {
        ServiceConfigurationError sce = new ServiceConfigurationError(service.getName() + ": " + msg);
        sce.initCause(cause);
        throw sce;
    }

    private static void fail(Class service, String msg) throws ServiceConfigurationError {
        throw new ServiceConfigurationError(service.getName() + ": " + msg);
    }

    private static void fail(Class service, URL u, int line, String msg) throws ServiceConfigurationError {
        fail(service, u + ":" + line + ": " + msg);
    }

    /**
     * Parse a single line from the given configuration file, adding the name on the line to both the names list and the
     * returned set iff the name is not already a member of the returned set.
     */
    private static int parseLine(Class service, URL u, BufferedReader r, int lc, List<String> names, Set<String> returned)
            throws IOException, ServiceConfigurationError {
        String ln = r.readLine();
        if (ln == null) {
            return -1;
        }
        int ci = ln.indexOf('#');
        if (ci >= 0) {
            ln = ln.substring(0, ci);
        }
        ln = ln.trim();
        int n = ln.length();
        if (n != 0) {
            if (ln.indexOf(' ') >= 0 || ln.indexOf('\t') >= 0) {
                fail(service, u, lc, "Illegal configuration-file syntax");
            }
            int cp = ln.codePointAt(0);
            if (!Character.isJavaIdentifierStart(cp)) {
                fail(service, u, lc, "Illegal provider-class name: " + ln);
            }
            for (int i = Character.charCount(cp); i < n; i += Character.charCount(cp)) {
                cp = ln.codePointAt(i);
                if (!Character.isJavaIdentifierPart(cp) && cp != '.') {
                    fail(service, u, lc, "Illegal provider-class name: " + ln);
                }
            }
            if (!returned.contains(ln)) {
                names.add(ln);
                returned.add(ln);
            }
        }
        return lc + 1;
    }

    /**
     * Parse the content of the given URL as a provider-configuration file.
     *
     * @param service The service class for which providers are being sought; used to construct error detail strings
     * @param u The URL naming the configuration file to be parsed
     * @param returned A Set containing the names of provider classes that have already been returned. This set will be
     * updated to contain the names that will be yielded from the returned <tt>Iterator</tt>.
     * @return A (possibly empty) <tt>Iterator</tt> that will yield the provider-class names in the given configuration file
     * that are not yet members of the returned set
     * @throws ServiceConfigurationError If an I/O error occurs while reading from the given URL, or if a configuration-file
     * format error is detected
     */
    @SuppressWarnings({ "empty-statement" })
    private static Iterator<String> parse(Class service, URL u, Set<String> returned) throws ServiceConfigurationError {
        InputStream in = null;
        BufferedReader r = null;
        ArrayList<String> names = new ArrayList<>();
        try {
            in = u.openStream();
            r = new BufferedReader(new InputStreamReader(in, "utf-8"));
            int lc = 1;
            // noinspection StatementWithEmptyBody
            while ((lc = parseLine(service, u, r, lc, names, returned)) >= 0) {
                ;
            }
        } catch (IOException x) {
            fail(service, ": " + x);
        } finally {
            try {
                if (r != null) {
                    r.close();
                }
                if (in != null) {
                    in.close();
                }
            } catch (IOException y) {
                fail(service, ": " + y);
            }
        }
        return names.iterator();
    }

    /**
     * Private inner class implementing fully-lazy provider lookup
     */
    private static class LazyIterator<T> implements Iterator<T> {
        final Class<T> service;
        final ClassLoader loader;
        Enumeration<URL> configs = null;
        Iterator<String> pending = null;
        final Set<String> returned = new TreeSet<>();
        String nextName = null;
        URL currentConfig = null;

        private LazyIterator(Class<T> service, ClassLoader loader) {
            this.service = service;
            this.loader = loader;
        }

        @Override
        public boolean hasNext() throws ServiceConfigurationError {
            if (nextName != null) {
                return true;
            }
            if (configs == null) {
                try {
                    String fullName = prefix + service.getName();
                    if (loader == null) {
                        configs = ClassLoader.getSystemResources(fullName);
                    } else {
                        configs = loader.getResources(fullName);
                    }
                } catch (IOException x) {
                    fail(service, ": " + x);
                }
            }
            while (pending == null || !pending.hasNext()) {
                if (!configs.hasMoreElements()) {
                    return false;
                }
                currentConfig = configs.nextElement();
                pending = parse(service, currentConfig, returned);
            }
            nextName = pending.next();
            return true;
        }

        @Override
        public T next() throws ServiceConfigurationError {
            if (!hasNext()) {
                throw new NoSuchElementException();
            }
            String cn = nextName;
            nextName = null;
            try {
                return service.cast(Class.forName(cn, true, loader).newInstance());
            } catch (ClassNotFoundException x) {
                fail(service, "Provider " + cn + " is specified in " + currentConfig + " but not found");
            } catch (Exception x) {
                fail(service, "Provider " + cn + " is specified in " + currentConfig + "but could not be instantiated: " + x, x);
            }
            return null; /* This cannot happen */
        }

        @Override
        public void remove() {
            throw new UnsupportedOperationException();
        }
    }
}
